It is often desirable to increase the rate at which a rubber is vulcanized. By doing so the length of time required to cure a rubber article is reduced. Increased vulcanization rates accordingly lead to greater throughputs and decreased energy requirements in curing operations. Even modest increases in vulcanization rate can result in greatly increased productivity and substantial cost savings in the curing of large rubber articles, such as tires.
It is very common to include accelerators in rubber formulations in order to increase the rate at which the rubber vulcanizes. Such compounds which increase the rate at which rubbers sulfur vulcanize are widely known to persons skilled in the art and are frequently referred to as primary accelerators. Some representative examples of compounds that can be utilized as primary accelerators include thiazoles, sulfenamides, dithiocarbamates, and thiurams. In many cases the primary accelerator is used in conjunction with another accelerator, called an activator or secondary accelerator, which further increases the rate of sulfur vulcanization.
The vulcanization accelerators presently used throughout the industry present a wide variety of different problems. For instance, dithiocarbamates and thiurams, which are sometimes called "ultra-accelerators", rapidly accelerate vulcanization at conventional vulcanization temperatures, but frequently lead to the premature vulcanization of the rubber composition during processing steps, such as mixing or molding. In other words, the utilization of dithiocarbamate accelerators and thiuram accelerators frequently reduces the scorch resistance of the rubber. Sulfenamide accelerators usually do not present scorch problems, but they tend to promote slower rates of sulfur vulcanization.
Tetramethylthiuram disulfide is widely used throughout the rubber industry as both a primary accelerator and a secondary accelerator. In fact, tetramethylthiuram disulfide has an outstanding combination of properties for use as a secondary accelerator. Unfortunately, it has been reported to be mutagenic in Ames testing. For this reason it would be desirable to find a replacement for tetramethylthiuram disulfide which acts in a similar manner as a secondary accelerator but which tests Ames negative.